This invention relates to a pneumatic tire, and more particularly, to a pneumatic tire in which the straight running stability is improved by reducing the ply steer that is conspicuous in radial tires in general, and in which the tire weight is reduced without spoiling the characteristics inherent to radial tires, such as their driving stability, high-speed durability, and load durability.
A prior-art radial tire for a passenger car usually has a construction in which at least two belt-reinforcing layers are provided over a carcass cord layer of a tread portion, substantially parallel to the circumferential direction of the tire. The reinforcing cords of the belt-reinforcing layer are inclined at an angle of 15.degree. to 30.degree. with respect to the circumferential direction of the tire, and cross each other. The carcass cord layer consists of one or two layers, and the edges of each layer are turned up around bead wires. The cords are arranged at an angle of about 90.degree. to the circumferential direction of the tire. A radial tire of this kind is superior to a bias tire with respect to braking performance, low fuel consumption, and wear resistance, thanks to the belt-reinforcing layers described above. The problem with the radial tire is, however, that its stability when driven in a straight line is poor because of the belt-reinforcing layers. As the radial tire rotates and moves forward, a lateral force occurs either to the right or left of the forward direction, even if the slip angle is zero. This lateral force makes the car move in a direction different to that intended by the driver.
In general, the lateral force when the slip angle is zero consists of components of forces generated by two different mechanisms, one is referred to as "connicity" (CT) and the other "ply steer" (PS). They are classified as part of the uniformity characteristics of the tire. The connicity (CT) and ply steer (PS) can be expressed by the following formulas from their definition in accordance with the uniformity testing method (JASO C607) for car tires: EQU LFDw=PS+CT (1) EQU LFSs=PS-CT (2)
where LFD is the mean value of the lateral forces experienced while the tire rotates once, LFDs is the mean value measured at the outer (or serial) side of the tire, and LFDs is the mean value measured when the tire is turned inside out.
PS and CT can be obtained as follows from formulas (1) and (2): EQU CT=(LFDw-LFDs)1/2 (3) EQU PS=(LFDw+LFDs)1/2 (4)
The relationship of formulas (1) through (4) can be illustrated diagrammatically as in FIG. 1.
Of the connicity and ply steer described above, connicity is believed to be a force generated because the tire shape is geometrically asymmetric about the center of the circumferential direction of the tire, that is, a force generated when a tire in the shape of a truncated cone rolls. The main reason for the generation of this force is the influence of the positions of the belt-reinforcing layers inserted into the tread of the tire. Hence the force can be reduced by improving the manufacture procedure. In contrast, ply steer is a force inherent to and arising from the structure of the belt-reinforcing layers themselves, and hence can not be reduced much in practice unless the structure of the belt-reinforcing layers is changed.
Now consider a belt-reinforcing layer. It can be represented as a two-layer laminated sheet 50 consisting of belt-reinforcing layers 50u, 50d, as illustrated in FIG. 2(A). It is well known that when a tensile force is made to act upon this two-layer laminated sheet 50 in the circumferential direction of the tire EE', the two-layer laminated sheet 50 undergoes deformation, not only in the two-dimensional plane in which the tensile force acts, but also three-dimensionally outside the plane, so that torsioned deformation like that shown in FIG. 2(B) occurs. The ply steer mentioned above occurs because of torsional deformation of the belt-reinforcing layers.
In the past, various studies have been done to determine whether this ply steer could be reduced by adding an extra belt-reinforcing layer or layers to the existing belt-reinforcing layers. The addition of extra belt-reinforcing layers is not preferable because it adversely affects the characteristics of the radial tire such as its low fuel consumption characteristics, and increases the tire weight.